[training] feat: Add Bridge support for standalone MTP pipeline stage

[yaoyu-33]

Summary

• Adds an opt-in DeepSeek-V3 layout helper mode that places MTP layers in a standalone penultimate PP/VPP stage and leaves loss in the final stage.
• Exports the DeepSeek-V3 layout helper and documents standalone MTP usage with pipeline_model_parallel_layout / m stages.
• Adds targeted unit coverage for default colocated MTP placement and standalone layout validation against Megatron Core layout parsing.

Context

• Tracks Megatron-LM PR [sync] clear aux losses tracker after eval, add cuda graph validation #2136: feat(moe): Support placing MTP layers into standalone stages NVIDIA/Megatron-LM#2136
• No Megatron Core submodule changes are included; the pinned submodule already contains mtp_standalone, LayerType.mtp, mtp_on_this_rank, get_mtp_ranks, and m layout docs.

Validation

• git diff --check
• uv run --active --no-sync ruff check src/megatron/bridge/recipes/deepseek/__init__.py src/megatron/bridge/recipes/deepseek/deepseek_v3.py tests/unit_tests/recipes/test_deepseek_recipes.py
• uv run --active --no-sync ruff format --check src/megatron/bridge/recipes/deepseek/__init__.py src/megatron/bridge/recipes/deepseek/deepseek_v3.py tests/unit_tests/recipes/test_deepseek_recipes.py
• uv run --active --no-sync pre-commit run --all-files

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[yaoyu-33]

/ok to test ad50a53

[claude]

Nit: this filter works today but is fragile — if a future export contains _config in its name but isn't a recipe builder (e.g., get_default_config_overrides), it would silently enter the parametrized test and likely fail in a confusing way. Consider matching on a suffix like name.endswith("_config") or name.endswith("_config_32nodes"), or adding a _RECIPE_NAMES set for explicit control.

[claude]

Light Code Review

Clean PR — the new mtp_standalone path is well-isolated, existing behavior is unchanged, and the two unit tests validate the happy path against Megatron Core's PipelineParallelLayerLayout parser.

Minor — test filter fragility (inline comment posted on test_deepseek_recipes.py:38)
The _config in name filter that replaced the old callable(...) filter could silently pick up future non-recipe exports whose names happen to contain _config. An explicit name set or stricter suffix match would be more robust.

Missing test coverage worth adding

• _build_standalone_mtp_layout with total_stages < 3 — verify the ValueError is raised.
• _build_standalone_mtp_layout with mtp_layers=0 — verify the ValueError is raised.
• mtp_standalone=True with mtp_num_layers > 1 (e.g., 3) — confirm multiple MTP entries appear in the penultimate stage.
• layout explicitly provided alongside mtp_standalone=True — confirm layout takes precedence and mtp_standalone is ignored.

Suggested test cases

No perf tests impacted.

[yaoyu-33]

/ok to test 47acb24

[yaoyu-33]

/ok to test 74dd4f7

[yaoyu-33]

DEEPSEEK_V3_NUM_DECODER_LAYERS I don't think we need this global variable, you should always be able to num layers